1. Field of the Invention
The present invention relates to a vehicle-to-vehicle communication system for performing wireless communication using white space, and more particularly to a vehicle-to-vehicle communication system in which frequency selection is performed by acquiring white space information from a database apparatus storing the white space information.
2. Description of the Related Art
To improve the efficiency of frequency utilization, research is being undertaken into cognitive radio, in which a wireless communication apparatus recognizes and acknowledges a peripheral radio wave environment and modifies a frequency, a radio system, and so on used for wireless communication adaptively. In particular, frequencies allocated to but not actually used by a licensed user (a primary user) may be used by an unlicensed user (a secondary user). These frequencies are referred to as secondarily usable frequencies, white space, or the like. To use this white space, the secondary user must detect the usable frequencies and determine a frequency that can be used favorably.
A method of performing spectrum sensing may be employed as a method of detecting a usable frequency. By performing spectrum sensing, a frequency utilization condition in the vicinity of a current location can be ascertained. With spectrum sensing, however, the frequency utilization condition in a location apart from the current location cannot be ascertained, and therefore a mobile device must perform continuous sensing. When continuous sensing is performed, a quiet period (transmission stop period) may be provided to differentiate between primary communication and secondary communication, but as a result, a reduction in channel utilization efficiency occurs.
A method employing a white space database has been proposed as a method that avoids the problems inherent in spectrum sensing (Patent Document 1 below, for example). The white space database manages information indicating whether or not a frequency is in use by a primary user in relation to each location and each frequency. In response to an enquiry (a query) specifying position information, the white space database provides frequency information relating to the vicinity of the specified position. A mobile device such, as a vehicle preferably issues a new query every time the device moves by a predetermined distance, and the Federal Communication Commission (FCC) requires that the database be accessed every 100 m. When a vehicle moves at 100 km per hour, the vehicle must access the database once every 3.6 seconds. However, this exchange is unlikely to be completed in time. Moreover, when the database is accessed from a large number of vehicles, a communication network may become congested.